1. Technical Field of the Invention
The present invention relates to a calcium removal method, and more particularly to a method of removing calcium contained in filtrate and others, which are obtained by washing chlorine bypass dust collected by a chlorine bypass system attached to a cement manufacturing facility.
2. Description of the Related Art
Chlorine bypass systems have been used to remove chlorine that may cause troubles such as preheater clogging in cement manufacturing facilities. In recent years, recycling of waste through conversion to cement raw material or fuel has been promoted, which increases the quantity of volatile matters such as chlorine, and the quantity of chlorine bypass dust generated as the quantity of the treated waste increases, which necessitates developments of methods for effectively utilizing the chlorine bypass dust.
From the above-mentioned point of view, in a treatment method through conversion to cement raw material described in Patent Document 1, to wastes containing chlorine is added water to allow the chlorine in the wastes to be eluted and filtrated; desalted cake obtained is utilized as a cement raw material; and waste water is purified and is discharged as it is or salt is recovered from the waste water, which makes it possible to effectively utilize the chlorine bypass dust without causing environmental pollution.
However, in this method, when desalting the chlorine bypass dust, in order to reduce selenium concentration in waste water to a safety level, for instance, to 0.1 mg-Se/l when discharged to sewage, ferrous chloride (FeCl2) as a reducing agent is required more or equal to 8000 mg-Fe2+/l, so that a large amount of reduction agent is consumed to remove selenium, resulting in a problem of skyrocketing operation cost.
Then, in order to resolve the above-mentioned problems, the applicant proposed, in Patent Document 2, a method of treating dust contained in extracted cement kiln combustion gas as shown in FIG. 9.
The treating method roughly divided into a washing process for washing the chlorine bypass dust to remove chlorine component, a waste water treatment process for removing heavy metals such as selenium from a filtrate, and a salt recovery process for recovering salt from concentrated salt water to obtain industrial salt.
In the washing process, a hot water generated in a boiler 61 is fed to a dissolution tank 63 through a hot water tank 62 so as to be mixed with the chlorine bypass dust. With this, water soluble chlorine component contained in the chlorine bypass dust dissolves in the hot water. A slurry discharged from the dissolution tank 63 is solid-liquid separated in a belt filter 64, and a primary cake from which chlorine component is removed is returned to cement kilns or the like to utilize it as a cement raw material. On the other hand, a primary filtrate containing chlorine component and heavy metals such as selenium is temporally stored in a storage tank 65.
In the waste water treatment process, the primary filtrate, which is stored in the storage tank 65, containing chlorine component, heavy metals such as selenium, and calcium is supplied to a chemical liquid reaction tank 66, and hydrochloric acid as a pH adjuster is added thereto to adjust pH in the chemical liquid reaction tank 66 below or equal to 4. After reducing and depositing selenium as a heavy metal contained in the waste water with ferrous sulfate, calcium hydroxide is added thereto to increase pH between 8 and 11, and ferrous hydroxide generated by adding ferrous sulfate is flocculated and deposited.
Next, a slurry discharged from the chemical liquid reaction tank 66 is solid-liquid separated by a filter press 67, and the secondary cake is returned to cement kilns or the like to utilize it as a cement raw material, and the secondary filtrate is mixed with potassium carbonate in the chemical liquid reaction tank 68 to remove calcium in the secondary filtrate.
Next, a slurry discharged from the chemical liquid reaction tank 68 is solid-liquid separated by a filter press 69, and the tertiary cake is returned to cement kilns or the like to utilize it as a cement raw material, and the tertiary filtrate is adjusted in pH in a storage tank 70 by adding hydrochloric acid, and iron, residual heavy metals and suspending substances (SS) are removed by an iron removal tower 71, a chelating resin tower 72 and a filtering device 73.
A waste water from the filtering device 73 is fed to an electrodialysis device 74, and in the electrodialysis device 74, selenium acid (SeO42-) in the waste water is included in a desalted water, and chlorine component is included in a concentrated salt water. The desalted water from the electrodialysis device 74 is returned to the hot water tank 62 in the washing process through a circulation route not shown (refer to symbol A).
Further, in the salt recovery process, the concentrated salt water is heated in a heater 76 with vapor from the boiler 75 to perform crystallization in a crystallizer 77. In the crystallizer 77, solute in the concentrated salt water is deposited as crystals, through a centrifugal separator 80, industrial salt with potassium chloride as a main ingredient is recovered, which can be utilized as an industrial raw material. On the other hand, moisture which evaporated in the crystallizer 77 is cooled in a condenser 78 to recover drainage, which is returned to the washing process. A filtrate separated in the centrifugal separator 80 is returned to the crystallizer 77 through a filtrate tank 79. It is also possible to release the concentrated salt water without collecting salt therefrom.
Patent document 1: Japanese Patent Publication Heisei 11-100243 gazette.
Patent document 2: Japanese Patent Publication 2004-330148 gazette.
However, in the method of treating dust contained in extracted cement kiln combustion gas described in the Patent Document 2, after removing heavy metals in the chemical liquid reaction tank 66, in the chemical liquid reaction tank 68 is removed calcium in the secondary filtrate by adding potassium carbonate, the expense for adding the potassium carbonate increases, which causes a problem of skyrocketing operation cost.
the present invention has been made in consideration of the above problems in the conventional art, and the object thereof is to provide a calcium removal method and so on which makes it possible to decrease cost for chemicals to remove calcium and suppress operation cost.